The present invention relates generally to the field of pulleys such as may be used with the endless drive belt of a vehicle accessories drive system, and a method for producing the pulley. More particularly, the invention relates to i method of producing a pulley assembly, in which a plastic outer pulley body is molded about a metal insert by utilizing a locating means for positioning the metal insert inside the mold cavity, to maintain the roundness of the metal insert inside the plastic outer pulley body during molding and to ensure the concentricity of the belt engaging portion of the plastic outer pulley body relative to the round metal insert. Specifically, the invention relates to a method of producing a pulley assembly, in which a plastic outer pulley body is molded about a metal insert by utilizing a locating means for positioning the metal insert inside the mold cavity, and including a locking means on the outer flat circumferential surface of the metal insert to resist relative rotation between the plastic outer pulley body and the metal insert.
Pulleys are commonly used in vehicle accessory drive systems. Various vehicle accessories such as the power steering pump, alternator, air conditioning pump and water pump are operated by a single endless drive belt driven by a pulley connected to the engine crankshaft and linked to driven pulleys operatively associated with the accessories. This system is referred to as a "serpentine" drive belt system. Driving a number of accessories, particularly with a serpentine drive belt, requires careful handling of the belt to ensure proper placement of the belt on the driven pulleys operatively associated with the accessories. Precise location of the belt frequently calls for the use of idler pulleys. In addition, to prevent slippage of the belt, a constant load must be maintained on the endless drive belt system. The use of a tensioner assembly, complete with a tensioner arm attached to a tensioner pulley, helps maintain a load or torque on the endless drive belt system. Idler and tensioner pulleys are typically manufactured from steel, but the steel is expensive, heavy and can be difficult to form into a satisfactory pulley construction. These pulleys are also manufactured from nylon, but the nylon does not have good dimensional stability and the nylon pulley experiences excessive wear during operation.
Pulleys formed of plastic and metal are not new, per se; prior art plastic belt-driven pulleys with metal inserts exist of the type in which a plastic outer pulley body is both molded on and supported on a metal disc. U.S. Pat. Nos. 4,473,363 and 4,468,210 illustrate examples of the plastic outer pulley body molded on a metal disc in a one-step process. U.S. Pat. Nos. 4,913,688 and 5,476,423 illustrate examples of a two-step assembly process wherein first the plastic outer pulley body is molded and then the metal insert is placed into the already molded pulley body.
A number of plastic pulleys with metal inserts are manufactured by the one-step molding process. In the case of a pulley assembly manufactured by a compression molding process, a plastic ring shaped perform is heated until pliable, then cavity molded about the metal insert. A simple way to locate the metal insert within the mold cavity is a single locating pin positioned within the center hole of the metal disc. If the metal insert is a bearing assembly, preserving the concentricity or roundness of the metal bearing is of great importance. Because of the high pressure involved in the compression molding process, the use of a single locating pin through the center of the bearing has been found to result in loss of concentricity of the bearing within the plastic body of the pulley, rendering the bearing unacceptable. Issues that exist with a bearing that is out of round include shortened bearing life at high RPM and possible noise generation.
Traditionally, when producing a pulley by the one-step process of compression molding the plastic body about the metal insert, the outer periphery of the metal insert is physically altered (or upset). Examples include utilizing a metal insert with projections on its outer periphery, such as radially extending scallops, as described in U.S. Pat. No. 4,473,363, or with a knurl applied thereto, as described in U.S. Pat. No. 4,468,210. The outer periphery of the metal insert is altered (or upset) for the purpose of resisting relative rotation between the metal insert and the plastic outer body.
Many pulleys are also produced by the two-step manufacturing process. U.S. Pat. No. 4,913,688 illustrates an example of attaching the metal insert to the plastic outer pulley body after the body has been molded; to assist the bonding of the plastic to the metal, an adhesive, such as epoxy, is applied between the metal and plastic, after the plastic outer pulley body has been molded as a separate piece. U.S. Pat. No. 5,476,423 illustrates first molding a plastic outer pulley body, and while the plastic is still above 38.degree. C. (100.degree. F.), positioning a bearing within the plastic body and cooling the assembly, to allow the plastic body to shrink around and fix the bearing in place. The adhesive bonding method as well as the shrinkage method of attaching the metal insert to the plastic pulley body does overcome the risk of a loss of concentricity of the round metal insert (or bearing), but introduces other problems. The process of bonding with an adhesive presents clean-up and environmental problems. And the method of utilizing the plastic shrinkage to hold the metal insert in place does not, in practice, sufficiently lock the metal insert to the plastic body.
The need remains, particularly in the area of the endless drive belt of a vehicle accessories drive system, for a plastic pulley with a metal insert that may be manufactured easily and successfully by a process which maintains the roundness of the metal insert.